Method of checking forsterite, apparatus that evaluates forsterite, and production line that manufactures steel sheet

ABSTRACT

The location where forsterite is present is checked in a region from which light excited by an electron beam is emitted when a material containing forsterite is irradiated with an electron beam. The material is preferably a grain oriented electrical steel sheet having a forsterite layer. In addition, it is preferable that the accelerating voltage be 10 kV or more when an electron beam is radiated when the material is a grain oriented electrical steel sheet having a tension coating layer on the forsterite layer.

TECHNICAL FIELD

This disclosure relates to a method of checking forsterite, an apparatusthat evaluates forsterite, and a production line that manufactures asteel sheet.

BACKGROUND

A grain oriented electrical steel sheet is mainly used as an iron-corematerial for electric devices such as a transformer and so forth.Therefore, there is a demand for a grain oriented electrical steel sheethaving an excellent magnetization property, in particular, low ironloss.

Such a grain oriented electrical steel sheet is manufactured, forexample, by performing hot rolling on a steel slab containing aninhibitor (source thereof) necessary for secondary recrystallization,for example, MnS, MnSe, and AlN, by performing hot band annealing asneeded, by thereafter performing cold rolling one time, or two times ormore with intermediate annealing interposed between the performances ofcold rolling to obtain a final thickness, thereafter performingdecarburization annealing, thereafter applying an annealing separatorsuch as MgO to the surface of the steel sheet, and thereafter performingfinal finishing annealing. A forsterite (Mg₂SiO₄)-based insulationcoating (forsterite layer) is formed on the surface of such a grainoriented electrical steel sheet other than in exceptional cases.

The forsterite layer effectively contributes to a decrease in the amountof eddy current by electrically insulating stacked steel-sheet layersfrom each other when the steel sheets are used in the form of stackedlayers. However, when the forsterite layer on the surface of a steelsheet is non-uniform or when the flaking of the forsterite layer occurswhen a wound core is manufactured, there is a decrease in commodityvalue. In addition, there is a decrease in lamination factor, and alsolocal heat generation occurs due to a decrease in insulation propertywhich is caused by pressure generated when the wound core is assembled,which results in an accident in a transformer.

In addition, such forsterite layer is not formed only for the purpose ofelectric insulation. Since it is possible to give tensile stress to asteel sheet by utilizing the low thermal expansion of a forsteritelayer, a forsterite layer contributes to improvement in iron loss and,further, in magnetostriction. Further, such forsterite layer contributesto an improvement in magnetic property through purification of the steelby absorbing the constituents of an inhibitor which are no longer neededinto the forsterite layer when secondary recrystallization has beencompleted. Therefore, obtaining a uniform and smooth forsterite layer isone of the important points influencing the product quality of a grainoriented electrical steel sheet.

In addition, when the amount of forsterite is excessively large,point-like defects in which flaking occurs locally in a forsteritelayer, tend to occur in general. On the other hand, when the amount offorsterite is excessively small, there is a decrease in adhesionproperty with, for example, a steel sheet. To date, therefore, theamount of forsterite formed (the amount of forsterite) and thedistribution morphology of forsterite have been given importance. Inaddition, since it is necessary to control the amount and distributionmorphology of forsterite to manufacture a grain oriented electricalsteel sheet, it is very important to evaluate these factors.

Examples of techniques for investigating the amount of forsterite andthe distribution of the amount of forsterite include the following. Oneis a method in which the amount of forsterite is determined byperforming oxygen analysis on the surface of a steel sheet.Specifically, since a tension coating layer is usually formed on aforsterite layer to further improve the magnetic properties, thistension coating layer is removed first, then, steel is dissolved and,then, the amount of oxygen is determined using an infrared absorptionmethod after combustion.

In addition, examples of methods of checking the distribution of aforsterite layer include one in which a surface from which the tensioncoating layer has been removed is observed using a scanning electronmicroscope (SEM). In that case, elemental analysis may be conducted bydetecting characteristic X-rays.

In addition, examples of methods of investigating the distribution in across section include one in which the cross section of a steel sheet isprepared by performing, for example, polishing and in which the crosssection is observed using a SEM (for example, Japanese Unexamined PatentApplication Publication No. 2012-36447).

However, the methods described above all involve destructive analyses.In addition, any of such analyses takes a long time to complete theevaluation and prepare samples. Moreover, there is currently no methodof even checking the presence of forsterite easily without destroying ameasurement object.

It could therefore be helpful to provide a technique to easily check thepresence of forsterite without destroying the measurement object.

In addition, it could be helpful to provide a technique to easily checkthe location where forsterite is present without destroying themeasurement object.

In addition, it could be helpful to provide a technique to check theamount of forsterite and distribution of the amount of forsterite in anon-destructive manner, in a field of view wide enough to represent theobject, and quantitatively.

SUMMARY

We thus provide:

(1) A method of checking forsterite, the method including checking thelocation where forsterite is present in a region from which lightexcited by an electron beam is emitted when a material containingforsterite is irradiated with an electron beam.

(2) A method of checking forsterite, the method including checking theamount of forsterite and/or the distribution of the amount of forsteritein an unknown material containing an unknown amount of forsterite usingthe signal intensity or brightness of the light which is excited by anelectron beam and emitted when the unknown material is irradiated withthe electron beam based on the correlation between the amount offorsterite and the signal intensity or brightness of the light which isexcited by an electron beam and emitted when a material containingforsterite is irradiated with the electron beam.

(3) The method of checking forsterite according to item (1) or (2), inwhich the material is a grain oriented electrical steel sheet having aforsterite layer.

(4) The method of checking forsterite according to item (3), in whichthe material is a grain oriented electrical steel sheet having a tensioncoating layer on the forsterite layer, and in which the accelerationvoltage is 10 kV or more when the surface of the tension coating layeris irradiated with the electron beam.

(5) The method of checking forsterite according to any one of items (1)to (4), in which forsterite is checked using light having a wavelengthof 560 nm or more among the light which is excited by the electron beamand emitted.

(6) An apparatus that evaluates forsterite, the apparatus including asample stage for holding a material containing forsterite, anelectron-beam-radiation device for irradiating the material with anelectron beam, a light-evaluation device for evaluating light which isexcited by the electron beam and emitted when the electron beam isradiated from the electron-beam-radiation device, and a vacuum chamberin which the stage, the electron-beam-radiation device and thelight-evaluation device are arranged.

(7) The apparatus that evaluates forsterite according to item (6), theapparatus further including a wavelength cut filter for passing lighthaving a wavelength of 560 nm or more placed between theelectron-beam-radiation device and the light-evaluation device.

(8) The apparatus that evaluates forsterite according to item (6) or(7), in which the light-evaluation device includes a light-measurementunit for measuring the signal intensity or brightness of light which isexcited by the electron beam and emitted when the material is irradiatedwith the electron beam from the electron-beam-radiation device, acorrelation-storage unit for storing the correlation between the signalintensity or the brightness and the amount of forsterite, and aquantitative-analysis unit for deriving the amount of forsterite and/orthe distribution of the amount of forsterite in an unknown materialcontaining an unknown amount of forsterite based on the signal intensityor brightness of the light which is measured using the light-measurementunit when the unknown material is irradiated with the electron beam andthe correlation stored in the correlation-storage unit.

(9) A production line that manufactures a grain oriented electricalsteel sheet having a forsterite-formation section in which a forsteritelayer is formed on the grain oriented electrical steel sheet, theproduction line including an electron-beam-radiation device forirradiating the grain oriented electrical steel sheet having theforsterite layer with an electron beam, a light-evaluation device forevaluating light which is excited by the electron beam and emitted whenthe grain oriented electrical steel sheet is irradiated with theelectron beam from the electron-beam-radiation device, and a vacuumregion which is placed downstream of the forsterite-formation sectionand in which the electron-beam-radiation device and the light-evaluationdevice are arranged.

(10) The production line that manufactures the steel sheet according toitem (9), the production line further including a wavelength cut filterfor passing light having a wavelength of 560 nm or more placed betweenthe electron-beam-radiation device and the light-evaluation device.

(11) A method of checking forsterite, the method including checkingwhether or not forsterite is present in a material based on whether ornot light excited by radiation of an electron beam is emitted from thematerial when the material is irradiated with the electron beam.

(12) A method of checking forsterite, the method including checking theamount of forsterite in an unknown material containing an unknown amountof forsterite using the emission intensity of light which is excited byan electron beam and emitted when the unknown material is irradiatedwith the electron beam based on the correlation between the amount offorsterite and the emission intensity of the light which is excited byan electron beam and emitted when a material containing forsterite isirradiated with the electron beam.

It is thus possible to easily check the presence of forsterite withoutdestroying the measurement object.

It is also possible to easily check the location where forsterite ispresent without destroying the measurement object.

It is further possible to evaluate the amount of forsterite anddistribution of the amount of forsterite in a non-destructive manner, ina field of view wide enough to represent the object, and quantitatively.In particular, when the distribution of the amount of forsterite ischecked, it is possible to easily check whether or not the forsteritelayer is uniform and smooth. “Uniform” refers to when there is only alittle variation in the distribution of forsterite depending on thelocation, and “smooth” refers to when there is only a little variationin coating weight depending on the location.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an example of a forsteriteevaluation apparatus.

FIG. 2 is a schematic diagram illustrating a light-evaluation devicewhich is arranged in the forsterite evaluation apparatus illustrated inFIG. 1.

FIG. 3 is a diagram illustrating the secondary electron image (top part)of the cross section of a sample and the CL image (bottom part) of thesame field of view as used for the secondary electron image.

FIG. 4 is a diagram illustrating the secondary electron image observedfrom the surface of a sample similar to that observed in FIG. 3.

FIG. 5 is a diagram illustrating the CL image of the same field of viewas used for the secondary electron image in FIG. 4.

FIGS. 6(a) to 6(d) are diagrams illustrating the secondary electronimages and CL images obtained by using two kinds of samples havingdifferent adhesion properties between a forsterite layer and a grainoriented electrical steel sheet.

FIGS. 7(a) and 7(b) are diagrams illustrating the CL images which areobtained by binarizing the CL images in FIG. 6.

FIG. 8 is a graph produced by plotting average CL luminance against theamount of oxygen in a coating along the horizontal axis.

FIG. 9 is a diagram illustrating an example of a CL spectrum obtainedfrom the surface of a grain oriented electrical steel sheet with anaccelerating voltage of 25 kV.

FIG. 10 is a diagram illustrating the relationship between the amount ofoxygen in the coating and CL luminance when a wavelength cut filter isused.

FIG. 11 is a diagram illustrating the variation of CL luminance withrespect to a temperature in a forsterite-formation process.

REFERENCE SIGNS LIST

-   -   1 forsterite evaluation apparatus    -   10 sample stage    -   11 electron-beam-radiation device    -   12 light-evaluation device    -   120 light-measurement unit    -   121 quantitative-analysis unit    -   122 correlation-storage unit    -   13 vacuum chamber    -   14 wavelength cut filter    -   2 sample

DETAILED DESCRIPTION

We found that light is emitted from a forsterite layer when the surfaceof a steel sheet is irradiated with an electron beam. This light iselectron-beam-excitation light, that is, cathodoluminescence (CL).However, although this CL has been well known in the past and used inthe field of, for example, semiconductor materials (for example, TakashiSekiguchi: Materia Japan, vol. 35, pp. 551-557 (1996)), it is previouslyunknown that forsterite formed on the surface of electrical steel sheetsexhibits CL.

In addition, there has been no concept regarding light being emittedfrom forsterite when the forsterite formed on the surface of anelectrical steel sheet is irradiated with an electron beam. Moreover,when a sample is a grain oriented electrical steel sheet having atension coating layer, a tension coating layer having a thickness ofseveral μm is present on a forsterite layer. Therefore, in particular,in a grain oriented electrical steel sheet having a tension coatinglayer, it could not be conceived that light is emitted from a forsteritelayer due to an electron beam.

We clarified the following by equipping a SEM with a light-evaluationdevice (light-evaluation device including, for example, alight-detecting device), scanning and irradiating the surface and crosssection of a grain oriented electrical steel sheet with an electronbeam, and performing CL image observation in which an image is producedusing the light signal of the excited light.

It is possible to visually check that forsterite is present in thesample based on CL generated in forsterite.

It is possible to obtain the CL image of a forsterite layer even if atension coating layer is formed on the forsterite layer when observationis performed from the surface of a grain oriented electrical steelsheet.

The amount of CL signal (such as signal intensity or brightness)obtained from electron-beam-excited light emitted from a forsteritelayer approximately correlates to the amount of forsterite.

It is possible to derive the distribution of the forsterite layer of agrain oriented electrical steel sheet based on a CL image obtained fromelectron-beam-excited light emitted from the forsterite layer.

In addition, we found that the CL in the forsterite layer of anelectrical steel sheet has two or more peaks in a visible light range.Therefore, we found that it is possible to extract certain informationregarding the forsterite layer by selecting the light to be detectedusing an optical filter. For example, by detecting red light, there isan increase in the degree of the correlation between signal intensity orbrightness and the amount of forsterite.

Moreover, we found that, by first obtaining a CL image and by digitizingthe luminance of the CL image instead of directly detecting the lightintensity of electron-beam-excited light emitted from a forsteritelayer, it is possible to check the amount of forsterite and thedistribution of the amount of forsterite quantitatively and easily.

Details of our methods, apparatus and production lines follow below.

First, the material (hereinafter, also referred to as “sample”) will bedescribed. By using our methods, it is also possible to check that asample does not contain forsterite. Therefore, the meaning of “samplewhich is used in our methods” includes not only one containingforsterite but also one not containing forsterite.

When a sample does not contain forsterite, it is only possible to checkthat the sample does not contain forsterite. On the other hand, in asample containing forsterite, it is possible to check the presence offorsterite, the location where forsterite is present, the amount offorsterite, and the distribution of the amount of forsterite. Whenmaterials that generate CL other than forsterite are included, examplesof a method of distinguishing forsterite from other materials includeone based on emission intensity and one based on wavelength. However, itis preferable that materials that generate CL other than forsterite notbe included.

A grain oriented electrical steel sheet having a forsterite layer andtension coating layer may be used as a sample. Specifically, examples ofthe sample include a grain oriented electrical steel sheet having aforsterite layer, and a layered body having a layered structureincluding a tension coating layer, a forsterite layer, and a grainoriented electrical steel sheet in this order from the surface side.When these grain oriented steel sheets are used as the samples, since ingeneral the forsterite layer contains mainly Mg₂SiO₄, and since thetension coating layer contains, for example, a phosphate, the sampledoes not contain materials which generate CL other than forsterite.

Examples of a method of forming a forsterite layer on a grain orientedelectrical steel sheet include the following method. First,decarburization annealing (doubles as recrystallization annealing) isperformed on a grain oriented electrical steel sheet having a finalthickness and containing an appropriate amount of Si. Subsequently, anannealing separator (one containing mainly MgO is suitable) is appliedto the steel sheet, then the steel sheet is coiled, and the coiled steelsheet is subjected to final finishing annealing for the purpose ofsecondary recrystallization and the formation of a forsterite layer. Anoxide layer (subscale) containing mainly SiO₂ is formed on the surfaceof the steel sheet when the decarburization annealing is performed, andthis oxide layer reacts with MgO in the annealing separator when thefinal finishing annealing is performed. A forsterite layer (Mg₂SiO₄) isformed on the grain oriented electrical steel sheet by this reaction.

Examples of a method of forming a tension coating layer include one offorming a tension coating layer on a forsterite layer using an inorganiccoating method or a ceramic coating method such as a physical vapordeposition method or a chemical vapor deposition method, after finalfinishing annealing has been performed. By forming a tension coatinglayer, it is possible to decrease iron loss.

Subsequently, a forsterite evaluation apparatus for the method ofchecking forsterite according to our methods will be described. FIG. 1is a schematic diagram illustrating an example of a forsteriteevaluation apparatus. FIG. 2 is a schematic diagram illustrating alight-evaluation device arranged in the forsterite evaluation apparatusillustrated in FIG. 1. As illustrated in FIG. 1, a forsterite evaluationapparatus 1 includes a sample stage 10, an electron-beam-radiationdevice 11, a light-evaluation device 12, a vacuum chamber 13, and awavelength cut filter 14. As illustrated in FIG. 1, the sample stage 10,the electron-beam-radiation device 11, the light-evaluation device 12,and the wavelength cut filter 14 are arranged in the vacuum chamber 13.The degree of vacuum that can be realized by the vacuum chamber is adegree in which a SEM can function, and it is usually the degree ofvacuum of about 10⁻² Pa, or less than 10⁻² Pa. However, this is notalways applied to a system having differential evacuation is excluded.For example, our method can be realized even in the degree of vacuum upto about 200 Pa in such system.

Although the forsterite evaluation apparatus 1 has the wavelength cutfilter 14, it is possible to check the target information such as theamount of forsterite based on the information regarding light evenwithout the wavelength cut filter 14. Therefore, the wavelength cutfilter 14 is not necessarily equipped.

In the forsterite evaluation apparatus 1, it is possible to irradiate asample 2 held on the sample stage 10 with an electron beam (the electronbeam is denoted by a dotted arrow line) from the electron-beam-radiationdevice 11 (for example, including an electron-beam-generation device andan electron optical system to narrow the electron beam and scanning thesample). When the sample 2 irradiated with an electron beam containsforsterite, the sample 2 emits light excited by the electron beam. Byevaluating this luminescence using the light-evaluation device 12, it ispossible to check the presence of forsterite, the location whereforsterite is present, the amount of forsterite, and the distribution ofthe amount of forsterite. The forsterite evaluation apparatus 1 has thewavelength cut filter 14. By using the wavelength cut filter 14, it ispossible to evaluate light having a wavelength in a certain range amongelectron-beam-excited light using the light-evaluation device 12. Asdescribed below, there is an increase in the precision of the check byusing light having a wavelength of 560 nm or more.

As illustrated in FIG. 2, the light-evaluation device 12 includes alight-measurement unit 120, a quantitative-analysis unit 121, and acorrelation-storage unit 122. The light-evaluation device 12 is a deviceobtained by combining the correlation-storage unit 122, in which acertain correlation is stored, and the quantitative-analysis unit 121,with which quantitative analysis is performed on the information from alight detector based on the correlation described above, with a commonlight detector, with which light is detected and with which the signalintensity or brightness of the light is measured. Therefore, thepreferable light-evaluation device 12 is obtained, for example, bycombining a computer which has an ordinary quantitative analysisfunction and in which the correlation described above is stored with acommon light detector.

There is no particular limitation on the light-measurement unit 120 aslong as the device can detect visible light, and examples of the deviceinclude one which detects light using, for example, a photomultipliertube (PMT). In addition, the light-measurement unit 120 has a functionof converting information regarding the detected light into informationsuch as signal intensity or brightness. Therefore, when the sample 2 isirradiated with an electron beam from the electron-beam-radiation device11, the light-measurement unit 120 detects light excited by the electronbeam and emitted, and converts the information regarding this light intoinformation such as signal intensity or brightness. As described above,it is possible to check the presence of forsterite by checking thedetection of light from the light-measurement unit 120.

In addition, the light-measurement unit 120 can detect the light excitedby the electron beam and emitted, for each region when the surface ofthe sample 2 is divided into plural regions. Therefore, it is alsopossible to check a location (region) where forsterite is present bychecking the detection of light by the light-measurement unit 120. Thereis no particular limitation on the area of the region described above,and the area may be adjusted in accordance with, for example, requiredprecision for the check.

By using the methods described above, it is possible to check thepresence and location of forsterite. In particular, since the check canbe done without destroying the sample, it is possible to check how theforsterite has been formed in the process of the formation of theforsterite layer. There is no particular limitation on what method isused for recognizing the information regarding light detected by thelight-measurement unit 120, it is possible to check the informationusing the light-evaluation device 12 in combination with a SEM.

As described above, it is possible to determine the signal intensity orbrightness of light using the light-measurement unit 120. This signalintensity or brightness is transferred to the quantitative-analysis unit121 and, in the quantitative-analysis unit 121, the amount of forsteriteand the distribution of the amount of forsterite in the sample arederived based on the information regarding light and the correlationwhich is stored in the correlation-storage unit 122 (correlation betweenthe amount of forsterite and the signal intensity or brightness of thelight excited by an electron beam and emitted, when a sample containingforsterite is irradiated with the electron beam). More specifically, theamount of forsterite in a specified region is derived from the signalintensity or the brightness and the correlation, and the distribution ofthe amount of forsterite is derived by combining the informationregarding the amounts of forsterite in plural regions. “Brightness”refers to the brightness of a CL image which is derived based on thesignal intensity of electron-beam-excited light, and brightness can beexpressed in terms of luminance.

By using the methods and apparatus described above, it is possible toderive the amount of forsterite and the distribution of the amount offorsterite in the sample. In particular, since the check can be donewithout destroying the sample, it is possible to check how theforsterite has been formed in the process of the formation of theforsterite layer. Therefore, it is possible to easily determine theconditions to form a forsterite layer having the amount and distributionof forsterite in the desired range. In addition, it is possible to checkinformation related to the amount of forsterite and the distribution ofthe amount of forsterite, for example, by using the light-evaluationdevice 12 and a SEM in combination as described above.

There is no particular limitation on what method is used to derive thecorrelation which is stored in the correlation-storage unit 122. Forexample, using plural samples containing different amounts of forsteritewith the amounts in the samples having been known, it is possible toderive the correlation by irradiating each of the samples with anelectron beam and by determining the signal intensity or brightness ofthe electron-beam-excited light.

We will further describe in detail an example in which a material havinga layered structure including a forsterite layer and a tension coatinglayer formed in this order on a grain oriented electrical steel sheet isused as a sample.

By irradiating a sample with an electron beam, and by detecting thelight emitted at that time, CL intensity (the intensity of theelectron-beam-excited light) is obtained. In addition, by scanning thesurface of a sample with a narrowed electron beam, and by determining CLintensity in synchronization with the scanning position, a CL image canbe obtained. When the sample described above is used, it is preferablethat the acceleration voltage of the incident electron be selected from0.1 kV to 100 kV.

In the sample in this example, it was checked that only a forsteritelayer exhibits CL as illustrated in FIG. 3 (“backing plate” in FIG. 3refers to “backing plate made of copper”). FIG. 3 is a diagramillustrating the secondary electron image (top part) of the crosssection of the sample in this example and the CL image (bottom part) ofthe same field of view as used for the secondary electron image. It isclarified that, in the cross sections illustrated in FIG. 3, CL is notexcited in the base steel sheet or in the tension coating, and that CLis excited only in the forsterite layer. A method of obtaining thesecondary electron image in FIG. 3 is as follows. Using an apparatuscomprising a detector formed of SEM SUPRA55-VP™ manufactured by CarlZeiss AG, a light collecting mirror, and PMT, observation was performedunder the condition that the accelerating voltage was 3 kV. As in thisexample, when observation is performed from a cross section since highspace resolution is necessary, it is advantageous to use a detectorhaving high sensitivity with a low acceleration voltage.

FIG. 4 is a diagram illustrating the secondary electron image observedfrom the surface of a sample similar to that observed in FIG. 3 (aforsterite layer was intentionally peeled off from a part of the surface(coating-layer-peeled-off portion)) (the observation was performed witha tension coating layer being formed on the surface). Using SEMSUPRA55-VP manufactured by Carl Zeiss AG and an ET detector, thesecondary electron beam image was obtained under the condition that theacceleration voltage was 30 kV. In addition, the CL image in FIG. 5 wasobtained using the same devices and conditions as used to obtain thesecondary electron image in FIG. 4 (the observation was similarlyperformed with a tension coating layer being formed on the surface) withthe exception that a light detector (not having a light collectingmirror) including a transparent glass pipe and PMT was used. In FIG. 5,since the coating-layer-peeled-off portions look dark, it is clarifiedthat a forsterite layer was removed. In addition, in FIG. 5, darkportions extending in streaks in the rolling direction are recognized.By observing the cross section of this portion using a focused ion beam(FIB) method, we found that a forsterite layer was absent there. Also,from the results of this observation of the cross section, we found thata large amount of forsterite is present in the portion which is brighterthan the surrounding portions. From the results described above, it isclarified that the CL image indicates the distribution of the amount offorsterite. The enlarged views in FIG. 5 illustrate the cross sectionsalong the dotted lines and illustrate the results of the analysis ondistribution of forsterite layers based on the results (SE) of the SEMobservation performed on the cross sections prepared by a FIB method andMg mapping (Mg) using an EDS. The cross sections were prepared(observed) in two portions, where one was prepared at the portion (soundportion) where a forsterite layer was observed in the CL image obtainedfrom the surface and the other was prepared at the portion where aforsterite layer was absent extending in the rolling direction(defective portion).

It is usually thought that, when a tension coating layer is formed on aforsterite layer, it is difficult to check the state of the forsteritelayer in a non-destructive manner. However, it is possible to check thestate of a forsterite layer without removing a tension coating layer.This is because, when an electron beam is radiated, the acceleratedelectrons penetrate through the tension coating layer on the upper sideto the forsterite layer. Therefore, to check a forsterite present undera tension coating as in this example, it is necessary to controlaccelerating voltage which is an electron beam radiating condition.Although the necessary accelerating voltage varies in accordance withthe kind and thickness of the tension coating layer, the acceleratingvoltage may appropriately be 10 to 60 kV when the thickness of aphosphate-based tension coating layer is 1 to 2 μm. Specifically, sincethe amount of excited light increases with increasing acceleratingvoltage, there is an increase in the amount of information, which isadvantageous for detection. However, since the higher the acceleratingvoltage, the more the electron beam spreads in a sample, there is adecrease in space resolution. In addition, since there is an increase inthe number of electron beams passing through a forsterite layer, thereis a decrease in emission intensity. It is appropriate to control theaccelerating voltage based on such guidelines and the thickness of atension coating.

FIG. 6 is a diagram illustrating the secondary electron images and CLimages of two samples having different adhesion properties between aforsterite layer and a grain oriented electrical steel sheet, where thesamples were similar to that described above (which was prepared byforming a forsterite layer and a tension coating layer in this order ona grain oriented electrical steel sheet). FIG. 6(a) and FIG. 6(b) arerespectively the secondary electron image and CL image of a samplehaving a low adhesion property, and FIG. 6(c) and FIG. 6(d) arerespectively the secondary electron image and CL image of a samplehaving a high adhesion property. The conditions to obtain the secondaryelectron images and the CL images are the same as those used to obtainthe secondary electron image in FIG. 4 and the CL image in FIG. 5. It isnot possible to check the difference in adhesion property using thesecondary electron images of FIG. 6(a) and FIG. 6(c). On the other hand,it is possible to check the difference in adhesion property using the CLimages of FIG. 6(b) and FIG. 6(d). Specifically, using FIG. 6(b) andFIG. 6(d), it is possible to check that the sample having a highadhesion property contains a larger amount of forsterite than the samplehaving a low adhesion property and that the sample having a highadhesion property has less portions extending in the rolling directionwhere a forsterite layer was absent than the sample having a lowadhesion property. That is to say, it is possible to judge whether ornot the adhesion property is satisfactory based on a CL image indicatingthe distribution of the amount of forsterite. Moreover, FIG. 7illustrates binarized CL images. The histograms in FIG. 7 illustratedistributions of the brightness of the CL images. The binarized CL imageof FIG. 7(a) corresponds to the CL image of FIG. 6(b), and the binarizedCL image of FIG. 7(b) corresponds to the CL image of FIG. 6(d). By usingbinarization, it is possible to check the difference between the sampleswith increased clarity. Product management can be done based on the graylevel of a CL image or the area ratio of defects which is checked usinga CL image, as described above. We believe that when the amount offorsterite is large, since there is an increase in the number of contactpoints between a steel sheet and a forsterite layer, there is anincrease in adhesion property.

As illustrated in the examples above (in particular, in FIGS. 3 through5), it is effective to observe secondary electron images along with CLimages at the same time, because it is possible to understand the wholeimage and shape of the sample.

Subsequently, we investigated whether or not it is possible toquantitatively check the amount of forsterite from the signal intensityof CL. Using six samples having different amounts of forsterite (checkedbased on the amount of oxygen attached), the CL image of each sample wasobtained. The CL images were obtained using the same apparatus andconditions used when FIG. 5 was obtained. The average brightness of eachCL image obtained was determined on a 256-level gray scale using imageprocessing software (Photoshop CS6). FIG. 8 was obtained by plottingaverage brightness (average luminance) against the amount of oxygenattached (the amount of oxygen in a coating) measured along thehorizontal axis. We believe that there is a relationship that can beapproximated using a quadratic function between both factors. Thecorrelation coefficient R² was 0.95. Also, from the results describedabove, the following facts are clarified.

First, it is possible to check the amount of forsterite formed on thesurface of a grain oriented electrical steel sheet from the lightinformation such as the signal intensity or brightness of CL.

Second, it is possible to check the distribution of the amount offorsterite formed on the surface of a grain oriented electrical steelsheet from the distribution of the light information regarding a CLimage. Specifically, by equipping a light-evaluation device with acorrelation-storage unit in which the correlation between the amount offorsterite and light information is stored, it is possible to check thedistribution of the amount of forsterite in a sample from the signalintensity or brightness measured by the light-measurement unit.

Third, although a CL image was obtained by scanning a sample with anarrowed electron beam in the investigations described above, it ispossible to check the amount of forsterite formed on the surface of asteel sheet by examining the intensity of light signal regarding thelight emitted as a result of radiating an electron beam to the surfaceof a steel sheet using a simpler device not having a scanning system.

Fourth, although the results described above were obtained using a SEM,it is clear that the amount of forsterite can be checked on line byplacing a vacuum path (vacuum range) in a part of the production linethat manufactures an electrical steel sheet and by arranging anelectron-beam-radiation device and a light-evaluation device in thevacuum path. “Vacuum” in the vacuum range refers to the same degree ofvacuum as the degree of vacuum which can be realized by the vacuumchamber described above.

The precision of the check described above is improved by detectinglight having a wavelength in a certain range among electron-beam-excitedlight. FIG. 9 is a diagram illustrating an example of a CL spectrumobtained from the surface of a grain oriented electrical steel sheetwith an accelerating voltage of 25 kV. The spectrum roughly has peaks ata wavelength of 400 nm and 650 nm. The CL luminance (the averagebrightness of a CL image) was determined for each of the four sampleshaving different amounts of forsterite when a short-wavelength cutfilter which blocks the transmission of light having a wavelength of 590nm or less was used, when a long-wavelength cut filter which mainlydetects light having a wavelength of 350 nm to 510 nm was used, and whena wavelength cut filter was not used (the results obtained respectivelyusing the short-wavelength cut filter and the long-wavelength cut filterare illustrated in FIG. 9, where the vertical axis on the right-handside is used for these results). FIG. 10 is a diagram illustrating therelationships between the amount of oxygen in the surface layer (theamount of oxygen in the coating) and average CL luminance in therespective cases (the vertical axis on the right-hand side is used whenthe short-wavelength cut filter was used). When the short-wavelength cutfilter was used, although there is a decrease in CL intensity, theinconsistent points indicated by white outlined arrows are eliminated,which results in the relationship between the two factors beingillustrated clearer. As described above, the amount of forsterite can bechecked more precisely by using a wavelength cut filter. That is to say,by using light having a wavelength of 560 nm or more as a target forevaluation with the peak around a wavelength of 400 nm being excluded,there is an increase in the precision of the check.

EXAMPLES Example 1

By scanning and irradiating three fields of view of 2.6 mm×1.7 mm withan electron beam having an accelerating voltage of 30 kV for each of thegrain oriented electrical steel sheets given in Table 1 (samples havinga forsterite layer and a phosphate-based tension coating layer in thisorder on the surface of a grain oriented electrical steel sheet), CLimages were obtained in the same conditions using a light detectorincluding a light guide and a PMT. The average luminance of the obtainedimages was analyzed on a 256-level gray scale using existing imageanalyzing software (Photoshop CS6) (the luminance of each field of viewis given in Table 1). The evaluation items were average quantitativeperformance, distribution quantitative performance, whether or not it ispossible to check a sample in a non-destructive manner(“non-destruction” in the Table), and time necessary for check. Theaverage quantitative performance and distribution quantitativeperformance are checked by the following method. The results are givenin Table 2 with mark 5 (observation method 5).

We checked average quantitative performance based on whether or not themeasurement results correlate as compared to a method in which a tensioncoating layer was peeled off to perform oxygen analysis (whencorrelation coefficient R² was 0.7 or more was judged as a correlation).In addition, we checked whether or not it was possible to obtain theaverage information for an area of 10 mm×10 mm or more. The evaluationcriteria were as follows.

“◯”: when the average information for an area of 10 mm×10 mm or more wasobtained and when results having correlation were obtained.

“x”: when the average information for an area of 10 mm×10 mm or more wasnot obtained, when results having correlation were not obtained, or whenany of both were not obtained.

Distribution quantitative performance was judged based on whether or notit was possible to observe the distribution of forsterite with a spaceresolution of 10 μm or less and whether or not it was possible todetermine the amount of forsterite with this resolution. The evaluationcriteria were as follows.

“◯”: when it was possible to observe the distribution of forsterite witha space resolution of 10 μm or less and when it was possible todetermine the amount of forsterite with this resolution.

“Δ”: when it was possible to observe the distribution of forsterite witha space resolution of 10 μm or less and when it was not possible todetermine the amount of forsterite with this resolution.

“x”: when it was not possible to observe the distribution of forsteritewith a space resolution of 10 μm or less.

TABLE 1 Amount of Oxygen CL Luminance (Arbitrary Unit) Sample in CoatingField of Field of Field of Standard No. (g/m²) View 1 View 2 View 3Average Deviation 1 2.53 83.8 83.5 79.6 82.3 2.3 2 2.76 93.8 97.7 96.596.0 2.0 3 2.65 93.9 92.1 92.6 92.9 1.0 4 2.80 102.9 101.4 100.2 101.51.4 5 3.04 114.8 126.9 107.4 116.4 9.8 6 3.10 147.7 142.9 144.1 144.92.5

Since the standard deviation of the fields of view 1 through 3 was smallenough compared to the average luminance (“small enough” refers to when(standard deviation/average luminance)×100% is 9% or less), it isclarified that the average luminance of the three fields was determinedwith good reproducibility. By deriving a standard curve from thecorrelation illustrated in FIG. 8 and using this standard curve, it waspossible to determine the amount of forsterite of an unknown sample in anon-destructive manner. Moreover, by converting the distribution of theluminance of a CL image into that of the amount of forsterite using thestandard curve, it was possible to show the in-plane distribution of theamount of forsterite. Although a CL image was obtained in this example,it is needless to say that the same thing can be done by monitoring theemission intensity of light by radiating a wide electron beam withoutobtaining the image.

Using the observation methods 1 through 4 below other than the CL imageobservation according to our methods, similar evaluations wereperformed. The evaluation results are given in Table 2. When a harmfulliquid was used for evaluation was indicated by the description in thecolumn of “Other”.

Observation method 1 (surface layer peeling and oxygen analysis): byremoving the tension coating layer of the sample described above byimmersing the sample in a alkali liquid, and determining the oxygenconcentration using an infrared absorption method after combustion, theamount of forsterite was calculated from the oxygen concentration. Theabove evaluation was performed based on the amount of forsterite and theobservation method.

Observation method 2 (surface layer peeling and SEM observation): byremoving the tension coating layer of the sample described above usingthe same method described above, the surface of the sample after thetension coating layer had been removed was observed using a SEM. Theabove evaluation was performed based on the observation results obtainedusing a SEM and the observation method.

Observation method 3 (steel sheet removing and SEM observation): byremoving the grain oriented electrical steel sheet of the sampledescribed above, the surface of the sample after the steel sheet hadbeen removed was observed using a SEM. The above evaluation wasperformed based on the observation results obtained using a SEM and theobservation method.

Observation method 4 (SEM observation of cross section): the crosssection prepared by cutting the plate-shaped sample described above inthe direction at a right angle to the surface was observed using a SEM.The above evaluation was performed based on the observation resultsobtained using a SEM and the observation method.

As indicated by Table 2, although our methods are simple methods thatcan be completed in a short time, it is possible to check the existenceof forsterite, the location where forsterite exists, the amount offorsterite, and the distribution of the amount of forsterite.

TABLE 2 Average Time (Representative) Distribution Taken QuantitativeQuantitative for Code Method Performance Performance Non-destructionConfirmation Other Note 1 Surface Layer Peeling ◯ X X 1 Day UsingHarmful Liquid Comparative Example and Oxygen Analysis 2 Surface LayerPeeling X Δ X 1 Day Using Harmful Liquid Comparative Example and SEMObservation 3 Steel Sheet Removing and X Δ X 1 Day Using Harmful LiquidComparative Example SEM Observation 4 SEM Observation of X Δ X 0.5 Days Comparative Example Cross Section 5 CL Image Observation ◯ ◯ ◯   1Minute Example

Example 2

Using two samples having different adhesion properties, where thesamples are similar to those used in Example 1, the results illustratedin FIG. 6 and FIG. 7 are obtained as described above. As describedabove, by observing the CL image, it was possible to check not only thedifference in the amount of forsterite but also the distribution of theamount of forsterite. In addition, it was possible to evaluate animportant property, that is, an adhesion property of coating from theamount of forsterite and the area ratio of portions where a forsteritelayer was absent.

Example 3

To investigate the formation state of a forsterite layer with respect toa heating temperature in a forsterite layer forming process, CL imageobservation was performed on steel sheets which had not been treated(before coating MgO, which is a raw material) and steel sheets to whichMgO had been applied and which had subsequently been heated at atemperature of 850° C. to 1050° C. in a laboratory. Using SEM SUPRA55 VPand an acceleration voltage of 30 kV, CL images at a magnification of 50times (based on the Polaroid (registered trade mark) film size) areobtained using fixed observation conditions. In this case, a wavelengthcut filter was not used. The average luminance of the whole CL imageswas determined. FIG. 11 is a diagram illustrating the variation of theaverage luminance of CL images with respect to the heating temperature.From the results, it is verified that a forsterite layer was scarcelyformed at a temperature lower than 850° C., formation of a forsteritebegan at a temperature between 850° C. and 950° C., and the amount offorsterite formed increased with increasing heating temperature in therange of 950° C. or higher. On the other hand, when evaluation isperformed on such a group of samples using conventional methods, sinceoxides other than forsterite are present along with forsterite on thesurface of a steel sheet, it is difficult to check the amount offorsterite formed using an ordinary method of, for example, analyzingthe amount of oxygen.

The invention claimed is:
 1. A method of checking forsterite comprisingchecking a location where forsterite is present in a region from whichlight excited by an electron beam is emitted when a material containingforsterite is irradiated with an electron beam.
 2. The method accordingto claim 1, wherein the material is a grain oriented electrical steelsheet having a forsterite layer.
 3. The method according to claim 2,wherein the material is a grain oriented electrical steel sheet having atension coating layer on the forsterite layer, and the accelerationvoltage is 10 kV or more when the surface of the tension coating layeris irradiated with the electron beam.
 4. The method according to claim3, wherein forsterite is checked using light having a wavelength of 560nm or more among the light excited by the electron beam and emitted. 5.The method according to claim 2, wherein forsterite is checked usinglight having a wavelength of 560 nm or more among the light excited bythe electron beam and emitted.
 6. The method according to claim 1,wherein forsterite is checked using light having a wavelength of 560 nmor more among the light which is excited by the electron beam andemitted.
 7. A method of checking forsterite comprising checking anamount of forsterite and/or distribution of the amount of forsterite inan unknown material containing an unknown amount of forsterite usingsignal intensity or brightness of light excited by an electron beam andemitted when the unknown material is irradiated with the electron beam,based on a correlation between the amount of forsterite and the signalintensity or brightness of the light excited by an electron beam andemitted when the material containing forsterite is irradiated with theelectron beam.
 8. The method according to claim 7, wherein the materialis a grain oriented electrical steel sheet having a forsterite layer. 9.The method according to claim 5, wherein the material is a grainoriented electrical steel sheet having a tension coating layer on theforsterite layer, and the acceleration voltage is 10 kV or more when thesurface of the tension coating layer is irradiated with the electronbeam.
 10. The method according to claim 9, wherein forsterite is checkedusing light having a wavelength of 560 nm or more among the lightexcited by the electron beam and emitted.
 11. The method according toclaim 8, wherein forsterite is checked using light having a wavelengthof 560 nm or more among the light excited by the electron beam andemitted.
 12. The method according to claim 7, wherein forsterite ischecked using light having a wavelength of 560 nm or more among thelight excited by the electron beam and emitted.
 13. An apparatus thatevaluates forsterite comprising: a sample stage that holds a materialcontaining forsterite, an electron-beam-radiation device that irradiatesthe material with an electron beam, a light-evaluation device thatevaluates light excited by the electron beam and emitted when thematerial containing forsterite is irradiated with the electron beam fromthe electron-beam-radiation device, and a vacuum chamber in which thestage, the electron-beam-radiation device and the light-evaluationdevice are arranged.
 14. The apparatus according to claim 13, furthercomprising a wavelength cut filter that passes light having a wavelengthof 560 nm or more placed between the electron-beam-radiation device andthe light-evaluation device.
 15. The apparatus according to claim 14,wherein the light-evaluation device comprises: a light-measurement unitthat measures the signal intensity or brightness of light excited by theelectron beam and emitted when the material is irradiated with theelectron beam from the electron-beam-radiation device, acorrelation-storage unit that stores the correlation between the signalintensity or the brightness and the amount of forsterite, and aquantitative-analysis unit that calculates the amount of forsteriteand/or the distribution of the amount of forsterite in an unknownmaterial containing an unknown amount of forsterite based on the signalintensity or brightness of the light measured using thelight-measurement unit when the unknown material is irradiated with theelectron beam and the correlation stored in the correlation-storageunit.
 16. The apparatus according to claim 13, wherein thelight-evaluation device comprises: a light-measurement unit thatmeasures the signal intensity or brightness of light excited by theelectron beam and emitted when the material is irradiated with theelectron beam from the electron-beam-radiation device, acorrelation-storage unit that stores the correlation between the signalintensity or the brightness and the amount of forsterite, and aquantitative-analysis unit that derives the amount of forsterite and/orthe distribution of the amount of forsterite in an unknown materialcontaining an unknown amount of forsterite based on the signal intensityor brightness of the light measured using the light-measurement unitwhen the unknown material is irradiated with the electron beam and thecorrelation stored in the correlation-storage unit.
 17. A productionline that manufactures a steel sheet having a forsterite-formationsection in which a forsterite layer is formed on a grain orientedelectrical steel sheet, the production line comprising: anelectron-beam-radiation device that irradiates the grain orientedelectrical steel sheet forming the forsterite layer with an electronbeam, a light-evaluation device that evaluates light excited by theelectron beam and emitted when the grain oriented electrical steel sheetis irradiated with the electron beam from the electron-beam-radiationdevice, and a vacuum region in which the electron-beam-radiation deviceand the light-evaluation device are arranged.
 18. The production lineaccording to claim 17, further comprising a wavelength cut filter thatpasses light having a wavelength of 560 nm or more placed between theelectron-beam-radiation device and the light-evaluation device.
 19. Amethod of checking forsterite comprising checking whether or notforsterite is present in a material based on whether or not lightexcited by radiation of an electron beam is emitted from the materialwhen the material is irradiated with the electron beam.
 20. A method ofchecking forsterite comprising checking an amount of forsterite in anunknown material containing an unknown amount of forsterite usingemission intensity of light excited by an electron beam and emitted whenthe unknown material is irradiated with the electron beam, based on acorrelation between the amount of forsterite and the emission intensityof the light excited by an electron beam and emitted when a materialcontaining forsterite is irradiated with the electron beam.